1. Field of the Invention
The disclosed information relates to an electrophotographic image forming apparatus such as a copying machine, a printer, or a facsimile machine.
2. Background Art
Electrophotography is a printing process in which an image typically is formed with toner. First, variable areas of electrostatic charge transfers the toner to paper and then heat or pressure may fix the transferred toner to the paper. A conventional electrophotographic image forming apparatus may bring a roller or blade charging member into contact with a photosensitive member of the image forming apparatus to charge the photosensitive member.
Two methods are available to charge a photosensitive member by using a contact charging method. The first method is an “AC charging method” and the second is a “DC charging method.” In the AC charging method, a superimposed voltage of direct-current voltage and alternating-current voltage is applied to a charging member to charge a photosensitive member. In the DC charging method, a direct-current voltage is applied to a charging member to charge a photosensitive member.
Due to the application of an alternating-current voltage, the AC charging method allows a more uniform charging of the surface of a photosensitive member than the DC charging method. However, the amount of discharge to the photosensitive member is larger in the AC charging method than the amount of discharge in the DC charging method. As a result, the surface of the photosensitive member is more easily scraped in the AC charging method. Thus, if a photosensitive member is charged using the AC charging method, the life of the photosensitive member will be shorter than when the photosensitive member is charged using the DC charging method.
The AC charging method has other disadvantages. For example, the AC charging method requires an AC power source, which means that the AC charging method has a greater initial cost and a higher running cost than that required for the DC charging method. In other words, the DC charging method is more cost effective than the AC charging method.
The DC charging method is not without its own issues. For example, the uniformity of surface potential of a photosensitive member (charging uniformity) is lower in the DC charging method than the charging uniformity in the AC charging method. Specifically, there has arisen a problem of stripe-shaped charging non-uniformity (charging lateral stripe) in the longitudinal direction (direction perpendicular to the circumferential direction) of an electrophotographic photosensitive member, which is caused by the non-uniform surface potential of the photosensitive member.
Japanese Patent Laid-Open No. 5-341626 discloses a configuration for suppressing formation of a charging lateral stripe that occurs when a photosensitive member is charged using the DC charging method. Specifically, among charging gaps produced by bringing a charging roller and a photosensitive drum into contact with each other, a charging gap located upstream in the rotation direction of the photosensitive member is irradiated with light (pre-nip exposure). Therefore, the charge of the photosensitive member is canceled in the charging gap located upstream, and the photosensitive member is charged in a charging gap located downstream in the rotation direction of the photosensitive member. In turn, this suppresses the occurrence of a charging lateral stripe caused by separating discharge.
There has been an increasing demand for electrophotographic devices adapted to form images on a variety of media. A configuration for forming images on various media by changing the process speed depending on the type of the medium has been widely adopted. When a toner image is fixed onto a paper having a relatively high basis weight (hereinafter referred to as “thick paper”), a large amount of heat is required to guarantee fixing properties equivalent to when a toner image is fixed onto plain paper (paper having basis weight of approximately 50 to 100 mg/m2). To account for the large amount of required heat when an image is to be formed on paper of high basis weight, the fixing speed of a fixing device typically is reduced to increase the heating time, and thus the amount of heat, given to the paper. Many image forming apparatuses further adopt a configuration in which the process speed of a photosensitive member also is reduced as, and in the same manner as, the fixing speed of the fixing device is reduced.
The above approaches have lead to further problems for an apparatus having a photosensitive member that moves at each process speed. For example, in such an apparatus, if a charging gap located upstream in the rotation direction of the photosensitive member is irradiated with constant light regardless of the process speed (rotational speed of the photosensitive member), then a charging lateral stripe occurs. Specifically, when an image is to be formed on plain paper, a charging lateral stripe occurs if the charging gap located upstream is exposed to light at an amount that is equal to the amount of light exposed to a charging gap located upstream when an image is to be formed on thick paper.